Some transmission type image display apparatuses such as liquid crystal display apparatuses employ a direct-type surface light source device as a backlight. In recent years, a direct-type surface light source device including a plurality of light emitting elements has been increasingly used as a light source.
A direct-type surface light source device has, for example, a substrate, a plurality of light emitting elements, a plurality of light flux controlling members (lenses) and a light diffusion member. The plurality of light emitting elements are disposed in a matrix on the substrate. Over each light emitting element, the light flux controlling member is disposed for expanding light emitted from each light emitting element in the surface directions of the substrate. The light output from the light flux controlling member is diffused by the light diffusion member, and planarly illuminates a member to be irradiated (e.g. a liquid crystal panel).
FIGS. 1A to 1C illustrate a configuration of a conventional light flux controlling member. FIG. 1A is a perspective view from the rear side, FIG. 1B is a cross-sectional perspective view from the rear side, and FIG. 1C is a cross-sectional view. In FIGS. 1A and 1B, legs formed on the rear side are not illustrated. As illustrated in FIGS. 1A to 1C, conventional light flux controlling member 20 includes incidence surface 22 on which light emitted from a light emitting element is incident and emission surface 24 for outputting the light entered from incidence surface 22 toward the outside. Incidence surface 22 is a surface with a recessed shape relative to the light emitting element and formed so as to face the light emitting surface of the light emitting element.
FIGS. 2A to 2C are illustrations of optical paths in light flux controlling member 20. FIG. 2A is an illustration of an optical path of a beam with emission angle 30°, FIG. 2B is an illustration of an optical path of a beam with emission angle 40°, and FIG. 2C is an illustration of an optical path of a beam with emission angle 50°. As used herein, “emission angle” (θ in FIG. 2A) means an angle of a beam relative to optical axis LA of light emitting element 10. Also in FIGS. 2A to 2C, legs formed on the rear side are not illustrated.
As illustrated in FIGS. 2A to 2C, the light emitted from light emitting element 10 enters the inside of light flux controlling member 20 from incidence surface 22. The light entered light flux controlling member 20 reaches emission surface 24, and is output toward the outside from emission surface 24 (solid arrow). At this time, the light is refracted according to the shape of emission surface 24, so that the traveling direction of the light can be controlled. On the other hand, part of the light reached emission surface 24 is reflected by emission surface 24 (Fresnel reflection) and reaches rear surface 26 facing the substrate on which light emitting element 10 is mounted (dashed arrow). When the light reached rear surface 26 is reflected by rear surface 26, excessive light travels in the direction directly above light flux controlling member 20 and therefore, luminance unevenness occurs. When the light reached rear surface 26 is output from rear surface 26, the light is absorbed into the substrate and therefore, the loss of light is large.
It is undesirable that the light reflected by emission surface 24 travel in the direction directly above light flux controlling member 20 or be absorbed into the substrate. PTL 1 proposes a light flux controlling member that can solve the above problems.
FIGS. 3A to 3C illustrate a configuration of a light flux controlling member disclosed in PTL 1. FIG. 3A is a perspective view from the rear side, FIG. 3B is a cross-sectional perspective view from the rear side, and FIG. 3C is a cross-sectional view. In FIGS. 3A and 3B, legs formed on the rear side are not illustrated. As illustrated in FIGS. 3A to 3C, in light flux controlling member 30 disclosed in PTL 1, annular inclining surface 32 is formed in rear surface 26. Inclining surface 32 is rotationally symmetric (circularly symmetric) about central axis CA of light flux controlling member 30, and inclined at a predetermined angle (e.g. 45°) relative to central axis CA.
FIGS. 4A to 4C are illustrations of optical paths in light flux controlling member 30. FIG. 4A is an illustration of an optical path of a beam with emission angle 30°, FIG. 4B is an illustration of an optical path of a beam with emission angle 40°, and FIG. 4C is an illustration of an optical path of a beam with emission angle 50°. Also in FIGS. 4A to 4C, legs formed on the rear side are not illustrated. As illustrated in FIGS. 4A to 4C, light reflected by emission surface 24 reaches inclining surface 32 in light flux controlling member 30. Then, part of the light reached inclining surface 32 is reflected by inclining surface 32 and travels in a lateral direction (see FIGS. 4A and 4B).
In this way, in light flux controlling member 30 disclosed in PTL 1, the light reflected by emission surface 24 does not easily travel in the direction directly above light flux controlling member 30 or is not easily absorbed into the substrate. Therefore, a light emitting device including light flux controlling member 30 disclosed in PTL 1 can radiate light more efficiently and uniformly than a light emitting device including conventional light flux controlling member 20.